Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion (“Li-ion”) batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (measured in Wh/kg) and energy density (measured in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.
When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity-increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, and Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials including BiF3 (303 mAh/g, lithiated) and FeF3 (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira, Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
Thus the advantage of using a Li metal negative electrode (sometimes referred to as an anode) is the much higher energy density of the entire cell, as compared to cells with graphitic or other intercalation negative electrode. A disadvantage of using pure Li metal is that lithium is highly reactive. Accordingly, the lithium metal has a propensity to undergo morphology changes, which cause structures having a high surface area to form on and around the negative electrode when the cell is being charged. Exemplary high surface area structures include dendrites and mossy structures.
Dendrites are the most common failure mode for cells with Li metal anodes. The dendrites form with a needle-like structure and can grow through the separator during charging of the cell, resulting in an internal short. “Soft shorts” that burn out rapidly result in a temporary self-discharge of the cell, while “strong shorts” consisting of a higher, more stable contact area can lead to complete discharge of the cell, cell failure, and even thermal runaway. While dendrites typically grow through the separator during charge, shorts can also develop during discharge depending on the external pressure placed on the cell and/or internal volume changes that occur in both the negative and positive electrodes.
Because Li metal is highly electronically conductive, the surface of the Li tends to roughen as the metal is plated and stripped. Peaks in the surface grow as dendrites during charge. During discharge, some smoothing of the dendrites occurs. Nonetheless, there is typically some roughness that remains at the end of discharge. Depending on the depth of discharge, the overall roughness can be amplified from one cycle to the next. Because the metal is essentially at the same electrochemical potential throughout, potential and, to a lesser extent, concentration gradients in the electrolyte phase drive the change in morphology.
Previous Li dendrite growth modeling work has shown that the moving front of a dendrite tends to accelerate during cell charge due to the higher current density localized at the dendrite tip relative to its base. Application of thermodynamic models has shown that dendrite initiation (i.e., initial roughening of an almost perfectly smooth surface) can be suppressed by applying mechanical stress and selecting solid electrolytes with shear moduli on the order of 10 GPa at room temperature. The same models indicate that surface tension at metal-fluid interfaces is insufficient to suppress dendrite initiation.
Related to dendrite initiation and growth is development of the Li morphology, which tends to increase the electrode surface area with cycling and consumes solvent to generate fresh passivation layers. Formation of high-surface-area mossy Li tends to occur during low-rate deposition from a liquid electrolyte, especially if the salt concentration is high. The high surface area combined with high reactivity of Li and flammability of the organic solvent makes for a very reactive and dangerous cell.
Because of the enormous challenge involved in stabilizing the Li surface chemically and mechanically through the use of electrolyte additives, such that passivation remains in effect over hundreds to thousands of cycles, the preferred treatment for rechargeable Li-based cells is the use of a solid-electrolyte membrane that is mechanically robust and chemically stable against both electrodes. Such a barrier removes several simultaneous constraints that the liquid electrolyte otherwise must satisfy, but the requirements for its properties are nonetheless multifaceted and challenging to obtain in a single material.
The barrier must be chemically stable with respect to some or all of the following: the liquid electrolyte in the positive electrode, electronic conductors and catalysts in the positive electrode, the metallic Li negative electrode, reactive species such as oxygen molecules and reaction intermediates, and (in aqueous cells) water. Solid electrolytes must also have sufficient Li+ conductivity over the operating temperature range of the cell, negligible electronic conductivity, and high elastic modulus to prevent Li dendrite initiation.
In order to reduce formation of lithium dendrites, internal shorts, electrolyte decomposition, and lithium morphology changes, a number of approaches involving solid electrolytes that conduct lithium ions but are electronically insulating have been attempted. One such approach involves the use of a poorly conducting amorphous material known as LiPON, which has been used successfully in thin film lithium-metal batteries. However, because of LiPON's low lithium conductivity, it is difficult to make cells with thick, high capacity electrodes and still maintain a desired rate of discharge.
Another approach involves the use of a block copolymer that includes lithium-conducting channels in a matrix of inactive polymer that has a high shear modulus, perhaps high enough to prevent lithium dendrite formation. This approach has several drawbacks: 1) the composite conductivity is too low at room temperature because the intrinsic conductivity of the conducting phase is low, and the high-shear-modulus phase does not conduct lithium ions, thus diluting the composite conductivity further; 2) polymers generally absorb liquids and therefore are not an effective barrier between lithium metal and liquid electrolytes in the positive electrode or separator; 3) Li-conducting polymers are typically unstable at high positive electrode potentials (>3.9 V vs. Li). Hence, lithium-metal cells with such polymer electrolytes are typically used without any liquid electrolyte in the positive electrode, and they are used with low-potential positive electrode materials, such as sulfur or LiFePO4.
What is needed, therefore, is a battery system that reduces the potential for dendrite formation and the undesired morphological changes in the anode of battery cells having metal anodes, and that enables the use of a high-potential positive electrodes to increase the overall energy density of the battery.